GEOS-6006: Animation doesn't work with REQUEST=GetMap

This issue is related to how the GeoServer OWS dispatcher parses requests coupled with how the animator is currently shimmed into the WMS service ("animate" is implemented as a request).  In the absence of HTTP GET KVP arguments for `Service` and `Request` the OWS dispatcher infers these arguments from the path used to generate the request. For example for a URI with the path `/wms/animate?...` the service is inferred as `wms` and the request inferred as `animate` *unless* overridden by a KVP argument. In the presence of a KVP `Request=GetMap` the `animate` request is overridden.  Since `animate` is intended to override `GetMap` this leads to undesired behavior.

The quick fix is to intercept calls to the animator and remove `Request=GetMap` (if present) before the dispatcher is called.  A long term fix may be drop the need for an explicit animation request and trigger the behavior from the requested output format (i.e. "image/gif;subtype=animate" which is specific to animation")

This pull request implements a interceptor to remove `Request=GetMap`, if present, from any call to the animate end point so that animate is successfully called.